Guillotine type cutting of steel tubing is extremely popular, particularly in the manufacture of welded-seam steel tubing, because of the speed and repetition rate with which the cutting operation may be performed. It has been known for many years that the formation of a dimple due to the partial collapse of the steel wall during the initial penetration of the guillotine blade may be avoided by cutting a transverse notch through the tube wall at the point of entry of the guillotine blade. See U.S. Pat. No. Re. 22,114 "Cutoff Mechanism" issued June 16, 1942 to Alexander Borzym which discloses an apparatus for simultaneously notching both top and bottom of the tube, thereby to prevent the tendency of the tube to roll during the notching operation. A later example is shown in U.S. Pat. No. Re. 30,025 a reissue of U.S. Pat. No. 4,036,091 issued July 19, 1977 to Alexander Borzym. In both of these reissued patents the notching operation is performed by a blade which is driven across the tube in a thrust direction; i.e. the blade is effectively pushed across the tube. A still later example is shown in U.S. Pat. No. 4,108,029 "Cutoff Die Set" issued Aug. 22, 1978 to Alexander Borzym. In this latter example, the notching blade, which may have several teeth of differing depth, is drawn or pulled across the tube axis from an initial position which lies directly in the path of the guillotine blade. A still further example of the latter type of notching mechanism is disclosed in U.S. Pat. No. 4,392,644 "Tube Cutting Apparatus" issued July 12, 1983 to John J. Borzym.
All of the patents listed above illustrate methods and apparatus for dimple free cutting of single wall tubing. It has been found that the straightforward application of the prior art techniques to the cutting of multiple wall tubing is not satisfactory; i.e. simply cutting a notch part way through the outermost tube wall results in a dramatic collapse of the innermost tube wall when the guillotine blade pushes through. Similarly, attempts to make a single notching cut which cuts fully through the outer wall and partially through the inner wall cannot produce satisfactory results. It is, therefore, the principal objective of the present invention to provide for the satisfactory dimple free guillotine style cutting of multiple wall tubing.